Abstract: With hot press simulator the microstructural changes of 15MnV steel in hot deformation and subsequent cooling was investigated. A methematical model for evoluation the microstructural changes is obtained. The model consists:austenite recrystallization kinetics, recrystallizied grain size, grain coasenning after recrystallization in single pass and multiple passes deformation under isothermal and continous cooling conditions, and the ferrite transformed from hot deformed austinite. Good agreement was obtained between the results predicted and measured.